This invention relates to the detection of downed electrical power lines used to distribute power in an electrical power system and, in particular, to the control of the voltage applied to, and the current flowing through, these power lines.
A problem which exists in detecting the nature of certain power line fault conditions will be explained with the aid of FIGS. 1 and 2. FIG. 1 illustrates a scheme for distributing power to, and along, a street lighting system. Typically the street lighting system includes a xe2x80x9cprimaryxe2x80x9d power line 21 (carrying, for example, 7.9 KV phase to ground) coupled via a disconnect switch (SW21) to the primary of a power transformer PT21. The secondary of power transformer PT21 is coupled via a contactor C21 to a power line 23 which is shown to branch off into a power line 25a and a power line 25a. Each one of the power line branches (e.g., 25a, 25b ) may extend, for example, from 100 feet to 3 (or more) miles. The contactor 21 is controlled by means of a photovoltaic sensor PC21 which closes the contactor C21 at night to energize the lighting system and which opens the contactor C21 during the day to de-energize the lighting system. In addition, there are fuses (FZ1, FZ2) along lines 25a and 25b, in series with the contactor C21, which in response to an overload condition are caused to xe2x80x9copenxe2x80x9d or xe2x80x9cblowxe2x80x9d when conducting currents ranging from 10 amperes to more than 100 amperes.
Typically, the power lines 23, 25a, 25b are run above ground, supported by poles. In the event any of the poles are damaged (e.g., during a storm, as a result of an automobile accident, or mechanical failure of the supporting structure) and/or due to lightning striking the power lines and/or their supporting structure, a power line may be broken (xe2x80x9crupturedxe2x80x9d) resulting in a short circuit condition or in an open circuit condition. If the break is such that there is a short circuit condition, an overload current may be drawn through the power line causing a fuse (e.g., FZ1, FZ2) to blow. However, if the break is such that the power line is either dangling in the air or contacting ground via a high impedance connection, little, if any, current is drawn. The disconnect switch SW21 would not be opened and the fuses (e.g., FZ1, FZ2) along the line would not be blown. Yet, the broken (xe2x80x9cdanglingxe2x80x9d) line presents a grave danger to human life. For, if a human, or an animal, makes contact with the xe2x80x9cdanglingxe2x80x9d broken power line serious injury or death may occur at currents as low as 0.05 amperes. Therefore, it is necessary to positively sense any xe2x80x9crupturedxe2x80x9d power line; particularly when the power line is xe2x80x9clivexe2x80x9d in order to remove the application of power to that line.
An open circuit along a power line may be sensed via monitors (e.g., M1, M2) located at the termination (end point) of each power line (e.g., ep1a, ep1b, ep2a, ep2b). If either (or both) of the monitors (M1 and/or M2) senses that no current flows through the monitor or that there is no voltage across the lines, the monitor can indicate that there is an open circuit along the line (i.e., no power being distributed). However, the fact that there is an electric xe2x80x9copen circuitxe2x80x9d along a particular power line does not necessarily mean that there is a physical break (xe2x80x9crupturexe2x80x9d) in the power line.
Referring to FIG. 1, note that there is a fuse FZ1 along line 25a inserted into the line to protect the line and equipment (not shown) against an overload condition. This fuse (e.g., FZ1) can blow and the monitor (e.g., M1) will indicate that there is an open circuit along that line (e.g., 25a ). However the fact that there is an open circuit (due to the fuse being blown) does not mean that the line is physically broken (ruptured or downed). Consequently, if, on the basis of what the monitor senses, it is assumed that the line is broken and the disconnect switch is opened, then power will be removed from line 25b as well as line 25a. This is highly undesirable since the disconnect switch controls the distribution of power to other branches and power to these branches should not be interrupted unless there is a real question of harm to human life. On the other hand, if there is a broken line, it is imperative that power to the line be removed.
The problem of detecting a broken power line has been illustrated using a xe2x80x9csecondaryxe2x80x9d distribution system (i.e., the relatively lower voltage for the street light line distribution scheme). A similar and perhaps more significant problem exists in detecting breaks in xe2x80x9cprimaryxe2x80x9d distribution systems (i.e., those distributing high voltages) as shown in FIG. 2.
FIG. 2 shows a substation 10 which is used to control the switching and distribution of electrical power from a generating source (e.g., 11a, 11b) to various distribution points and loads within a power system. Substation 10 includes a plurality of circuit breakers (e.g., CB1-CB9) which are coupled to the incoming transmission lines (e.g., PL1, PL2) and which enable the application of power to the power transformers (e.g., PT1, PT2) and to the outgoing feeders (i.e., conductors or power lines suchxe2x80x94f1-f4). The circuit breakers are used to disconnect (and hence protect) the respective power lines and/or pieces of equipment when a fault (e.g., an overload) condition is sensed. The switching (opening and closing) of the circuit breakers (e.g., CB1-CB9) within a substation is typically controlled by protective relays (e.g., REL1-REL 4) which, by controlling the opening and closing of the circuit breakers protect the incoming transmission lines, the power transformers, the outgoing feeders as well as the circuit breakers contained within the substation.
The protective relays used in the system may be any of a number of relays, including electromechanical, non-programmable static, and programmable microprocessor based relays. Each protective relay is generally designed to sense and/or respond to a fault condition based on a plurality of settings maintained in, or applied to, each relay. When a relay senses a fault condition it causes its associated circuit breaker(s) to interrupt the power to, or out, of the device(s) being protected.
As noted above, if a relay senses loss of power along one line and interprets the loss as a broken or downed line, power may be removed from lines servicing users who may suffer significant economic damage due to the power interruption. If, in fact, the loss of power is due to a broken line which is xe2x80x9clivexe2x80x9d, then the potential harm to life warrants the interruption of power distribution. However, if the loss of power is due to a blown fuse, then the disruption of power distribution is not warranted, Thus, when an open circuit condition is sensed, a problem exists in detecting whether the sensed open circuit is due to the line being broken and requiring additional removal of power, or to another cause (e.g., an open relay or a blown fuse) requiring no additional action.
The distribution of power along a power line may be interrupted by among others, a physical break (rupture) of the power line, the blowing of a fuse along the power line or the opening of a disconnect switch. Interruption of power distribution results in the interruption of current flow and in the absence of a voltage beyond the point where the power line is broken or the fuse is blown or the disconnect switch is opened. Applicant""s invention resides, in part, in the recognition that different conditions causing interruption in the power distribution provide different xe2x80x9csignaturesxe2x80x9d. For example, the characteristics of the voltage, current and power signals and their transient responses generated on a power line resulting from a rupture of the power line are different than the voltage, current and power signal characteristics generated and sensed due to the blowing of a fuse or the opening of a disconnect switch or a circuit breaker. The different responses generated (and sensed) represent different signatures (xe2x80x9ccharacteristicsxe2x80x9d) which may be used to positively identify the cause of a power interruption (e.g., whether it is due to a break in a power line).
Applicant""s invention also resides in the sensing of any disruption in the distribution of power along a power line and in sensing the signature of the signal on the power line at the time of the disruption of power, prior to opening a circuit breaker supplying power to that line.
An electrical power distribution system embodying the invention includes a first storage means for storing information pertaining to selected characteristics exhibited by a downed power line. The system also includes circuitry coupled to a power line for sensing selected characteristics of the power being distributed along the power line and for storing the sensed information in a second storage means. The system also includes circuitry coupled to the power line for sensing the presence of an open circuit on the power line and, when an open circuit condition is sensed, comparing information stored in the second storage means with information stored in the first storage means to determine whether the open circuit is due to a break in the power line.
Circuits embodying the invention include means for continuously sensing and monitoring the presence of power at the termination point of a power line and for communicating this condition to a receiver at the origination point of the power line. Whenever there is a loss of power, a signal condition indicative of power loss is generated. In accordance with the invention, at least one of the voltage, current and energy (power) characteristics present at the origination point of the power line is sensed and monitored. When a power loss signal is generated, the nature of at least one of the voltage, current and energy characteristics at the origination point of the power line at the time of the power loss is examined to determine whether a physical beak along the power line has occurred and to then trip (or open) the appropriate circuit breaker, sectionalizer, recloser, disconnector, or contactor and to do so quickly, securely and dependably.
The origination point of a power line as used herein and in the appended claims may be located at the transmission or distribution substation, or at a contactor controlling a street lighting circuit or at a switching point along the power distribution line such as a recloser, sectionalizer, or disconnector. An origination point may also be the physical area on a power line to which power is applied for subsequent distribution along the power line, or branches thereof. The termination point of the power line need not be the actual physical end of a line, but may include any point, or section, along the power line remote from the origination point.
In one embodiment of the invention, a first communication device is coupled to an origination point of a power line and a second communication device is coupled to a contact point along the power line between the origination point and a termination point of the power line; where the first and second communication devices can communicate with each other over the power lines by sending and receiving encoded messages. Whenever the distribution of power is disrupted between the origination and the contact points, a disrupt signal is produced indicative of the interruption and a possible break in the power line. In response to a disrupt signal, the first communication device inspects at least one of the various voltage, current and energy measurements made at the time of, or just prior to, the generation of the disrupt signal to determine whether the power line is indeed broken.
Where the power line has one origination point and several different termination points, due to branching of the power line, additional communication devices may be connected to each termination point, with each one of these second communication devices sending out a unique address for enabling the system to identify the affected power line segment.